Guide · The Company Tax Return
Since 31 March 2026 a company sends its Company Tax Return (CT600) to HMRC through commercial software. Here is what that software has to make, what you need ready, the dates, and how to send the return and your accounts together from the trial balance you already have.
What changed
HMRC and Companies House closed their free online service for company accounts and the tax return. From 1 April 2026 a company files its Company Tax Return, with its accounts, through commercial software - or has an accountant do it. A paper return is only for a reasonable excuse or a return in Welsh.
In GOV.UK's words, the software you choose "should be able to produce a CT600, a Corporation Tax computation and your company accounts". HMRC takes the accounts and the computation as inline XBRL files attached to the return, so the software makes those too.
The accounts still go to Companies House as well. Until April 2028 that can be through Companies House's own online service; from April 2028 every company files its accounts through software too.
What you need ready
The ten-digit Unique Taxpayer Reference on HMRC's letters to the company about Corporation Tax.
A trial balance at the year end, before closing entries, from your bookkeeping software or a spreadsheet, or the year's figures to type in - and last year's closing one for the comparatives, unless these are the company's first accounts.
The Government Gateway user ID and password the company uses for HMRC's online services, with Corporation Tax added - or an accountant's agent sign-in. Adding Corporation Tax for the first time needs an activation code HMRC sends by post, so allow for it.
The accounts the directors have approved and one has signed: the return carries them, with the income statement the Companies House filing copy can leave out.
The dates
Usually nine months and one day after the end of the accounting period - before the return is due. A company with a year to 31 March pays by 1 January.
Twelve months after the end of the accounting period it covers. A company with a year to 31 March 2026 files by 31 March 2027.
A late return brings a penalty that grows the later it is, and more if it is late again and again. The amounts are on GOV.UK.
With Stoneledger Accounts Filing
From Xero, QuickBooks, Sage or a spreadsheet, or type the year's figures in. Every line is placed and shown, and nothing goes on until both years balance.
In the format the company qualifies for, checked against the trial balance to the pound - free, before you pay anything.
The Corporation Tax worked out HMRC's way from the accounts: what tax adds back and takes off, capital allowances, losses, and loans to directors.
The CT600 by HMRC's own boxes, with the tax computation in HMRC's layout and the accounts attached, checked against HMRC's rules before anything is sent.
The accounts at Companies House with the company's authentication code, asked for at that moment and never kept. The return at HMRC with your HMRC sign-in. £8 once for the company's year.
Not for every return: a Company Tax Return that needs a supplementary page other than loans to participators (CT600A) - group relief or research and development relief, for instance - goes through other software.
Stoneledger Accounts Filing makes the accounts, the tax computation and the Company Tax Return from the trial balance in your software or the figures you type in, shows them free, and files them at Companies House and HMRC for £8 once for the year.
Sources (GOV.UK, read 29 October 2026): Company Tax Returns; Company Tax Returns: penalties for late filing; Filing company accounts and tax returns if you previously used the HMRC online service. This guide explains the rules; it is not tax or legal advice. Stoneledger is independent of HMRC and Companies House.